Related Articles

In the gardening world, some pest-control measures eliminate one problem while promoting another. Although some varieties of marigolds (Tagetes spp.) effectively control root-knot nematodes, all of them attract spider mites. If you plant marigolds around tomatoes, you may be luring tiny interlopers that attack your plants and compromise their health. Managing these pesky creatures helps ensure healthy tomato plants that bear succulent homegrown fruit.

Marigolds

A gardening myth persists that marigolds’ pungent odor repels insects. Although research shows that French dwarf varieties control root-knot nematodes, these are not insects but worms that live in soil and attack plant roots. For marigolds to have maximum effectiveness against nematodes, they must be sown in place or planted before flowers open and allowed to grow thickly for three to four months before being plowed into the soil. During these months, marigolds not only compete with tomato plants for water and nutrients, but they attract and harbor spider mites.

Spider Mites

Spider mites are tiny arachnids that find easy transport on air currents, which move them from marigolds to tomato plants. Although they are visible with the unaided eye, they are difficult to see without close scrutiny. Often, their presence is first made known by delicate webbing that they spin as defense mechanisms. Spider mites eat sap by piercing plant tissue and siphoning cells. They commonly form colonies, with many spider mites clustered on undersides of leaves.

Damage

As a spider mite feeds on plant cell sap, the mesophyll tissue collapses. This leaves a tiny, chlorotic dot at each point where the mite’s hypodermic-like mouthpiece pierces plant tissue. As the mite moves across a leaf’s surface, yellow stippling follows. Large mite populations can cause entire leaves to turn yellow and infestations lead to leaf drop. If enough leaves fall, plants are incapable of photosynthetic activity and they die from the inability to produce food to sustain themselves.

Control

Because spider mites reproduce so rapidly, early identification is critical to effectively treating them. All yellowing leaves are not symptomatic of spider mite damage, but the presence of delicate webbing on plants with yellow leaves usually confirms their presence. Although insecticides will kill these pests, they also eliminate predators that manage their populations naturally. For this reason, the University of Massachusetts Extension cautions that using broad-spectrum insecticides often exacerbates the problem and causes outbreaks of spider mites. Instead, you can dislodge them with jets of water from your garden hose or use horticultural soap to spray on plants to kill them. An organic control is rosemary oil, which deters spider mites but does not injure plants or beneficial predators.

About the Author

Victoria Lee Blackstone is a horticulturist and a professional writer who has authored research-based scientific/technical papers, horticultural articles, and magazine and newspaper articles. After studying botany and microbiology at Clemson University, Blackstone was hired as a University of Georgia Master Gardener Coordinator. She is also a former mortgage acquisition specialist for Freddie Mac in Atlanta, GA.